Light detection and ranging (lidar) systems have been developed that are capable of remotely measuring range-resolved wind speeds for use in various applications, including but not limited to weather forecasting, air quality prediction, air-traffic safety, and climate studies. Lidar systems have also been used in connection with remote sensing of chemical compounds, gases, and aerosol optical properties in the atmosphere, and surface chemistry and physical properties of hard targets. In general, lidar operates by transmitting light from a laser source to a volume or surface of interest and detecting the time of flight for the backscattered light to determine range to the scattering volume or surface.
A Doppler wind lidar also measures the Doppler frequency shift experienced by the light scattered back to the instrument due to the motions of molecules and aerosols (e.g. particles and droplets) in the atmospheric scattering volumes, which is directly tied to the speed of the wind in that volume, relative to the lidar line of sight (LOS). The wind speed along the LOS is determined by projecting the wind speed and direction (the wind vector) onto that LOS.
For large scale global weather forecasting, it is desirable to measure wind profiles over a wide range of atmospheric levels including lower atmospheric altitudes (e.g. the lower troposphere or up to approximately 2 km) where aerosols, including clouds (droplets), are more prevalent, and at higher altitudes (the upper troposphere and lower stratosphere, or 2 km-20 km) where aerosols may occasionally be present, but molecules are constantly present. Thus, for measuring full profile atmospheric winds, it is desirable to measure data from both aerosol returns and molecular returns. While both aerosol and molecular backscattered laser returns will see the same average wind-induced Doppler shift at their center frequency, aerosol backscattered returns have approximately the same illumination bandwidth as the laser light. Molecular backscattered returns, on the other hand, have a wide bandwidth due to the Doppler broadening induced by molecular vibration. Thus the two types of lidar returns will have different approaches to optimally estimating the change in center frequency.
Because molecular scattering is a function of wavelength to the inverse-fourth power, shorter wavelengths will scatter much more off molecules than longer wavelengths. The scattering ratio for aerosols is typically smaller. For this reason, molecular scatter wind lidar systems typically operate at the 355 nm wavelength, and aerosol scatter wind lidar systems can operate at longer wavelengths of 532 nm to 10 microns. Accordingly, hybrid lidar systems have been proposed that use 355 nm direct detection wind lidar for molecular returns, and 2 micron coherent detection wind lidar for aerosol returns. These hybrid systems thus require two different types of lasers, two different receivers, and expensive telescopes. In addition, such systems are bulky, have many points of potential failure, and are expensive to implement.
Another approach for measuring returns from both aerosols and molecules in the atmosphere combines a Fizeau spectrometer with a Fabry-Perot double-edge filter, both operating at the 355 nm wavelength. However, such filter-based systems have low receiver efficiencies, require significant laser and pointing stability, and still require multiple receivers, and therefore are relatively difficult and expensive to construct to obtain a desired sensitivity.